Effects of skin metabolism on percutaneous penetration of lipophilic drugs

The effect of alcohols as vehicles on the percutaneous absorption and skin retention of ibuprofen modified with l-valine alkyl esters

The effect of various alcohols as vehicles on skin permeability was compared for unmodified ibuprofen (IBU) and ion pairs of ibuprofen with l-valine alkyl esters [ValOR][IBU], in which the alkyl chain R was changed from C1 to C8. In vitro permeation experiments were conducted in a Franz cell with porcine skin. Methanol, ethanol, and isopropanol solutions of 70% (v/v) were chosen as vehicles for penetrants and a buffer solution of pH 5.4 or 7.4 as the acceptor phase. The comparisons of permeation profiles for various [ValOR][IBU] from different alcohols were determined. The cumulative mass, skin accumulation, steady-state flux, diffusion coefficient, and lag time were investigated and compared. It was observed that i-propanol was the best enhancer of skin permeation of both unmodified ibuprofen and its salts with l-valine alkyl esters for both acceptor phases. The permeability of the various carriers increases with increasing chain-length of the alcohol. In most cases, significantly higher cumulative mass was found in the acceptor buffer of pH 7.4. The conjugate of ibuprofen with l-valine propyl ester [ValOPr][IBU] permeated the skin to the highest degree in comparison to unmodified ibuprofen. The accumulation of ibuprofen was higher for all salts in relation to the parent acid applied onto the skin. The greatest amounts of ibuprofen were accumulated in the skin when ibuprofen was used as the ionic pair with l-valine butyl ester, [ValOBu][IBU] in the i-propanol solution and pH 7.4 buffer as the acceptor phase.

Isobutylparaben Negatively Affects Porcine Oocyte Maturation Through Increasing Oxidative Stress and Cytoskeletal Abnormalities

As a member of parabens (PBs), Isobutylparaben (IBP) has a broad-spectrum antimicrobial activity and widely used in personal care products and cosmetics. Recent studies have indicated that usage of IBP poses a potential threat to reproductive health. In this study, we aimed to reveal the effects of acute exposure to IBP on the meiotic maturation of porcine cumulus oocyte complexes. Initial study showed that 200 μM of IBP significantly reduced the rate of the first polar body extrusion with no significant effect on cumulus cell expansion; however, 400 μM of IBP could significantly affect both. Further research revealed that abnormal spindles, misalignment chromosomes, and aberrant distributed actin filaments were detected in IBP-treated oocytes, which indicates that the cytoskeleton architecture of oocyte could be the target of IBP. At the same time, ROS level and apoptosis rate of oocyte were significantly increased by IBP exposure. Moreover, the levels of H3K9me3 and H3K27me3 were significantly induced in oocytes by IBP. Collectively, these results demonstrate that acute exposure to IBP could disrupt porcine oocyte maturation through affecting cytoskeleton, oxidative stress, viability and epigenetic modification. Environ. Mol. Mutagen. 2020. © 2020 Wiley Periodicals, Inc.

Twenty Clinically Pertinent Factors/Observations for Percutaneous Absorption in Humans

At least 20 clinically relevant factors affect percutaneous absorption of drugs and chemicals: relevant physico-chemical properties, vehicle/formulation, drug exposure conditions (dose, duration, surface area, exposure frequency), skin appendages (hair follicles, glands) as sub-anatomical pathways, skin application sites (regional variation in penetration), population variability (premature, infants, and aged), skin surface conditions (hydration, temperature, pH), skin health and integrity (trauma, skin diseases), substantivity and binding to different skin components, systemic distribution and systemic toxicity, stratum corneum exfoliation, washing-off and washing-in, rubbing/massaging, transfer to others (human to human and hard surface to human), volatility, metabolic biotransformation/cutaneous metabolism, photochemical transformation and photosensitivity, excretion pharmacokinetics, lateral spread, and chemical method of determining percutaneous absorption.

Tissue-Specific Accumulation and Body Burden of Parabens and Their Metabolites in Small Cetaceans

Parabens have been of global concern due to their endocrine disrupting properties. However, few studies have reported tissue-specific distribution of parabens in wildlife. In this study, we measured parabens and their metabolites in organs and tissues (blubber, muscle, melon, stomach, kidney, liver, gonad, brain, uterus, and umbilical cord, total n = 94) of common dolphins ( Delphinus capensis) and finless porpoises ( Neophocaena asiaeorientalis), to investigate tissue-specific accumulation and body burden. Among the target compounds, methyl paraben (MeP) and para-hydroxybenzoic acid (4-HB) were detected in all organs. Compared to common dolphins, finless porpoises had significantly higher concentrations of MeP and 4-HB due to their near-shore habitat. Higher concentrations of MeP and 4-HB were found in the kidney, liver, and stomach than in other organs, indicating selective accumulation of parabens in certain organs. Significant correlations between MeP and 4-HB in liver/kidney suggested metabolic transformation of the former to the latter. Detection of parabens in brains, umbilical cords, and uteri suggests that these chemicals cross biological barriers such as the blood-brain and placental barriers. The body burdens of total parabens were in the ranges of 13000-90600 μg and 19800-81500 μg for common dolphins and finless porpoises, respectively.

Comparison of the metabolism of 10 chemicals in human and pig skin explants

Skin metabolism is important to consider when assessing local toxicity and/or penetration of chemicals and their metabolites. If human skin supply is limited, pig skin can be used as an alternative. To identify any species differences, we have investigated the metabolism of 10 chemicals in a pig and human skin explant model. Phase I metabolic pathways in skin from both species included those known to occur via cytochrome P450s, esterases, alcohol dehydrogenases and aldehyde dehydrogenases. Common Phase II pathways were glucuronidation and sulfation but other conjugation pathways were also identified. Chemicals not metabolized by pig skin (caffeine, IQ and 4‐chloroaniline) were also not metabolized by human skin. Six chemicals metabolized by pig skin were metabolized to a similar extent (percentage parent remaining) by human skin. Human skin metabolites were also detected in pig skin incubations, except for one unidentified minor vanillin metabolite. Three cinnamyl alcohol metabolites were unique to pig skin but represented minor metabolites. There were notable species differences in the relative amounts of common metabolites. The difference in the abundance of the sulfate conjugates of resorcinol and 4‐amino‐3‐nitrophenol was in accordance with the known lack of aryl sulfotransferase activity in pigs. In conclusion, while qualitative comparisons of metabolic profiles were consistent between pig and human skin, there were some quantitative differences in the percentage of metabolites formed. This preliminary assessment suggests that pig skin is metabolically competent and could be a useful tool for evaluating potential first‐pass metabolism before testing in human‐derived tissues.

We have investigated the metabolism of 10 chemicals in viable pig and human skin. Phase I and II metabolic pathways were present in skin from both species. Chemicals not metabolized by pig skin were also not metabolized by human skin. Six chemicals metabolized by pig skin were also metabolized to a similar extent by human skin. Pig and human skin produced common metabolites, although some species differences were observed and as their relative amounts differed.

Disconnecting the Estrogen Receptor Binding Properties and Antimicrobial Properties of Parabens through 3,5-Substitution

Commercially utilized parabens are employed for their antimicrobial properties, but a weak binding to the estrogen receptor alpha (ER_α) may lead to breast cancer in some applications. Modification of the paraben scaffold should allow for a disconnection of these observed properties. Toward this goal, various 3,5-substituted parabens were synthesized and assessed for antimicrobial properties against S. aureus as well as competitive binding to the ER_α. The minimum inhibitory concentration assay confirmed retention of antimicrobial activity in many of these derivatives, while all compounds exhibited decreased xenoestrogen activity as determined by a combination of competitive enzyme linked immunosorbent assay (ELISA), proliferation, and estrogen receptor binding assay. Thus, these changes to the paraben scaffold have led to a multitude of paraben derivatives with antimicrobial properties up to 16 times more active than the parent paraben and that are devoid or significantly diminished of potential breast cancer causing properties.

Interference of Paraben Compounds with Estrogen Metabolism by Inhibition of 17β-Hydroxysteroid Dehydrogenases

Parabens are effective preservatives widely used in cosmetic products and processed food, with high human exposure. Recent evidence suggests that parabens exert estrogenic effects. This work investigated the potential interference of parabens with the estrogen-activating enzyme 17β-hydroxysteroid dehydrogenase (17β-HSD) 1 and the estrogen-inactivating 17β-HSD2. A ligand-based 17β-HSD2 pharmacophore model was applied to screen a cosmetic chemicals database, followed by in vitro testing of selected paraben compounds for inhibition of 17β-HSD1 and 17β-HSD2 activities. All tested parabens and paraben-like compounds, except their common metabolite p-hydroxybenzoic acid, inhibited 17β-HSD2. Ethylparaben and ethyl vanillate inhibited 17β-HSD2 with IC₅₀ values of 4.6 ± 0.8 and 1.3 ± 0.3 µM, respectively. Additionally, parabens size-dependently inhibited 17β-HSD1, whereby hexyl- and heptylparaben were most active with IC₅₀ values of 2.6 ± 0.6 and 1.8 ± 0.3 µM. Low micromolar concentrations of hexyl- and heptylparaben decreased 17β-HSD1 activity, and ethylparaben and ethyl vanillate decreased 17β-HSD2 activity. However, regarding the very rapid metabolism of these compounds to the inactive p-hydroxybenzoic acid by esterases, it needs to be determined under which conditions low micromolar concentrations of these parabens or their mixtures can occur in target cells to effectively disturb estrogen effects in vivo.

In vitro skin absorption tests of three types of parabens using a Franz diffusion cell

The objective of this study was to evaluate the permeation of paraben derivatives - methylparaben (MP), propylparaben (PP), and butylparaben (BP) - in hairless mouse full skin and human cadaver epidermis using a Franz diffusion cell method, which is proposed as a reliable alternative method to an skin absorption test. Parabens, esterified hydroxybenzoic acid compounds, are widely used as preservatives in food, cosmetics, and pharmaceutical products. The skin permeation rate showed dose dependency, and the hairless mouse full skin showed a higher flux value than human cadaver epidermis. Among the permeability coefficient (K_p) values of three parabens, MP showed a higher K_p value than PP or BP. Hence, according to the definitions of Marzulli et al., parabens would be classified as "moderate" penetrants.

A proposed study on the transplacental transport of parabens in the human placental perfusion model

Human exposure to parabens as a preservative used in personal care products is of increasing concern, as there is evidence from in vivo and in vitro studies of hormone disruption in association with exposure to parabens. Transport across the placenta could be critical for risk assessment, but the available data are sparse. The aim is to develop a method for estimating fetal exposure, via the placenta, to the most commonly-used parabens, by using a human placental perfusion model. The use of human tissue is vital for determining human fetal exposure, because animal studies are of little relevance, since the placenta exhibits significant interspecies variation. An HPLC model is currently being established to simultaneously quantify four different parabens, namely, methylparaben, ethylparaben, propylparaben and butylparaben, and their main metabolite, p-hydroxybenzoic acid. With this model, we aim to determine the transport kinetics of these parabens across the human placenta, and to investigate placental metabolism, including differences in transport due to molecular characteristics. This will facilitate assessment of the risks associated with the use of paraben-containing products during pregnancy.

Biology of percutaneous penetration

BACKGROUND

Percutaneous penetration is a passive process that can occur following skin exposure to chemicals used in agriculture, industry, pharmaceuticals, cosmetics, and in the household. Once a penetrant is absorbed into the skin it may cause a local reaction or enter systemic circulation to produce widespread effects. Improved understanding of the skin barrier and biological aspects that impede absorption of topical substances is essential for advancing the fields of dermatotoxicology and dermatopharmacology as they pertain to percutaneous penetration.

METHODS

Pubmed search results for "percutaneous penetration", "absorption", "stratum corneum", "xenobiotics", "skin factors", "decontamination", and "transdermal" were reviewed from 1965 to 2014. Relevant articles discussing the influence of biological factors on percutaneous penetration of topical substances were included.

RESULTS

Absorption of a topical substance across the skin is most notably influenced by concentration, contact duration, frequency, and the surface area exposed. The interplay between these factors, along with skin biology and the physiochemical properties of the penetrant, can lead to enhanced percutaneous penetration.

CONCLUSION

Percutaneous penetration is a highly complicated and dynamic process influenced by numerous skin and environmental factors. Although research over the last few decades has provided plenty of new insights to improve our understanding of percutaneous penetration, many areas lack clarity due to conflicting data.

Dermal uptake of 18 dilute aqueous chemicals: in vivo disappearance-method measures greatly exceed in vitro-based predictions

Average rates of total dermal uptake (Kup ) from short-term (e.g., bathing) contact with dilute aqueous organic chemicals (DAOCs) are typically estimated from steady-state in vitro diffusion-cell measures of chemical permeability (Kp ) through skin into receptor solution. Widely used ("PCR-vitro") methods estimate Kup by applying diffusion theory to increase Kp predictions made by a physico-chemical regression (PCR) model that was fit to a large set of Kp measures. Here, Kup predictions for 18 DAOCs made by three PCR-vitro models (EPA, NIOSH, and MH) were compared to previous in vivo measures obtained by methods unlikely to underestimate Kup . A new PCR model fit to all 18 measures is accurate to within approximately threefold (r = 0.91, p < 10(-5) ), but the PCR-vitro predictions (r > 0.63) all tend to underestimate the Kup measures by mean factors (UF, and p value for testing UF = 1) of 10 (EPA, p < 10(-6) ), 11 (NIOSH, p < 10(-8) ), and 6.2 (MH, p = 0.018). For all three PCR-vitro models, log(UF) correlates negatively with molecular weight (r(2) = 0.31 to 0.84, p = 0.017 to < 10(-6) ) but not with log(vapor pressure) as an additional predictor (p > 0.05), so vapor pressure appears not to explain the significant in vivo/PCR-vitro discrepancy. Until this discrepancy is explained, careful in vivo measures of Kup should be obtained for more chemicals, the expanded in vivo database should be compared to in vitro-based predictions, and in vivo data should be considered in assessing aqueous dermal exposure and its uncertainty.

Modeling the human skin barrier--towards a better understanding of dermal absorption

Many drugs are presently delivered through the skin from products developed for topical and transdermal applications. Underpinning these technologies are the interactions between the drug, product and skin that define drug penetration, distribution, and elimination in and through the skin. Most work has been focused on modeling transport of drugs through the stratum corneum, the outermost skin layer widely recognized as presenting the rate-determining step for the penetration of most compounds. However, a growing body of literature is dedicated to considering the influence of the rest of the skin on drug penetration and distribution. In this article we review how our understanding of skin physiology and the experimentally observed mechanisms of transdermal drug transport inform the current models of drug penetration and distribution in the skin. Our focus is on models that have been developed to describe particular phenomena observed at particular sites of the skin, reflecting the most recent directions of investigation.

Effects of parabens on adipocyte differentiation

Parabens are a group of alkyl esters of p-hydroxybenzoic acid that include methylparaben, ethylparaben, propylparaben, butylparaben, and benzylparaben. Paraben esters and their salts are widely used as preservatives in cosmetics, toiletries, food, and pharmaceuticals. Humans are exposed to parabens through the use of such products from dermal contact, ingestion, and inhalation. However, research on the effects of parabens on health is limited, and the effects of parabens on adipogenesis have not been systematically studied. Here, we report that (1) parabens promote adipogenesis (or adipocyte differentiation) in murine 3T3-L1 cells, as revealed by adipocyte morphology, lipid accumulation, and mRNA expression of adipocyte-specific markers; (2) the adipogenic potency of parabens is increased with increasing length of the linear alkyl chain in the following potency ranking order: methyl- < ethyl- < propyl- < butylparaben. The extension of the linear alkyl chain with an aromatic ring in benzylparaben further augments the adipogenic ability, whereas 4-hydroxybenzoic acid, the common metabolite of all parabens, and the structurally related benzoic acid (without the OH group) are inactive in promoting 3T3-L1 adipocyte differentiation; (3) parabens activate glucocorticoid receptor and/or peroxisome proliferator-activated receptor γ in 3T3-L1 preadipocytes; however, no direct binding to, or modulation of, the ligand binding domain of the glucocorticoid receptor by parabens was detected by glucocorticoid receptor competitor assays; and lastly, (4) parabens, butyl- and benzylparaben in particular, also promote adipose conversion of human adipose-derived multipotent stromal cells. Our results suggest that parabens may contribute to obesity epidemic, and the role of parabens in adipogenesis in vivo needs to be examined further.

Three-dimensional skin models as tools for transdermal drug delivery: challenges and limitations

INTRODUCTION

Transdermal drug delivery has several known advantages over the oral route and hypodermic injections. The number of drugs that can be taken up transdermally is, however, limited owing to the innate barrier function of the skin. New transdermal drug candidates need to be tested extensively before being used on humans. In this regard, in vitro permeation methods are highly important to predict in vivo permeation of drugs.

AREAS COVERED

This review illustrates how different types of reconstructed skin models are being used as alternatives to human and pig skin for in vitro permeation testing of drugs. Insights into how various factors (including the physicochemical nature of molecules and formulations) or skin properties might affect the permeability of drugs in reconstructed skin models are provided. Also, opportunities and pitfalls of reconstructed skin models are highlighted.

EXPERT OPINION

Many studies have revealed that the permeability of reconstructed skin models is much higher compared with human excised skin. This is in accordance with the incomplete barrier found in these models. Nevertheless, the reconstructed skin models available today are useful tools for estimating the rank order of percutaneous absorption of a series of compounds with different physicochemical properties. A major challenge in the further development of reconstructed skin models for drug delivery studies is to obtain a barrier function similar to in vivo skin. Whether this goal will be achieved in the near future is uncertain and will be, in the authors' opinion, a very difficult task.

A new liquid chromatography-tandem mass spectrometry method for determination of parabens in human placental tissue samples

Endocrine disruptors are a group of organic compounds widely used, which are ubiquitous in the environment and in biological samples. The main effect of these compounds is associated with their ability to mimic or block the action of natural hormones in living organisms, including humans. Parabens (esters of p-hydroxybenzoic acid) belong to this group of compounds. In this work, we propose a new liquid chromatography-tandem mass spectrometry (LC-MS/MS) method to asses the presence of parabens most commonly used in industrial applications (methyl-, ethyl-, propyl- and butyl-paraben) in samples of human placental tissue. The method involves the extraction of the analytes from the samples using ethyl acetate, followed by a clean-up step using centrifugation prior to their quantification by LC-MS/MS using an atmospheric pressure chemical ionization (APCI) interface in the negative mode. Deuterated bisphenol A (BPA-d(16)) was used as surrogate. Found detection limits (LOD) ranged from 0.03 to 0.06 ng g(-1) and quantification limits (LOQ) from 0.1 to 0.2 ng g(-1), while inter- and intra-day variability was under 13.8%. The method was validated using standard addition calibration and a spike recovery assay. Recovery rates for spiked samples ranged from 82% to 108%. This method was satisfactorily applied for the determination of parabens in 50 placental tissue samples collected from women who live in the province of Granada (Spain).

Urinary concentrations of four parabens in the U.S. population: NHANES 2005-2006

BACKGROUND

Parabens are widely used as antimicrobial preservatives in cosmetics, -pharmaceuticals, and food and beverage processing.

OBJECTIVES

We assessed exposure to methyl, ethyl, propyl, and butyl parabens in a representative sample of persons >or= 6 years of age in the U.S. general population from the 2005-2006 National Health and Nutrition Examination Survey.

METHODS

We analyzed 2,548 urine samples by using online solid-phase extraction coupled to isotope dilution-high-performance liquid chromatography/tandem mass spectrometry.

RESULTS

We detected methyl paraben (MP) and propyl paraben (PP) in 99.1% and 92.7% of the samples, respectively. We detected ethyl (42.4%) and butyl (47%) parabens less frequently and at median concentrations at least one order of magnitude lower than MP (63.5 microg/L) and PP (8.7 microg/L). Least-square geometric mean (LSGM) concentrations of MP were significantly higher (p <or= 0.01) among non-Hispanic blacks than among non-Hispanic whites except at older ages (>or= 60 years). Adolescent and adult females had significantly higher (p < 0.01) LSGM concentrations of MP and PP than did adolescent and adult males. Females were more likely than males [adjusted odds ratios (ORs) and 95% confidence intervals (CIs): MP, 3.2 (2.99-5.27); PP, 4.19 (2.34-7.49)] and non-Hispanic blacks were more likely than non-Hispanic whites [MP, 4.99 (2.62-9.50); PP, 3.6 (1.86-7.05)] to have concentrations above the 95th percentile.

CONCLUSIONS

The general U.S. population was exposed to several parabens during 2005-2006. Differences in the urinary concentrations of MP and PP by sex and race/ethnicity likely reflect the use of personal care products containing these compounds.

Human skin permeation of 3-O-alkyl carbamate prodrugs of naltrexone

N-Monoalkyl and N,N-dialkyl carbamate prodrugs of naltrexone (NTX), an opioid antagonist, were synthesized and their in vitro permeation across human skin was determined. Relevant physicochemical properties were also determined. Most prodrugs exhibited lower melting points, lower aqueous solubilities, and higher oil solubilities than NTX. The flux values from N-monoalkyl carbamate prodrugs were significantly higher than those from NTX and N,N-dialkyl carbamates. The melting points of N-monoalkyl carbamate prodrugs were quite low compared to the N,N-dialkyl carbamate prodrugs and NTX. Heats of fusion for the N,N-dialkyl carbamate prodrugs were higher than that for NTX. N-Monoalkyl carbamate prodrugs had higher stratum corneum/vehicle partition coefficients than their N,N-dialkyl counterparts. Higher percent prodrug bioconversion to NTX in skin appeared to be related to increased skin flux. N,N-Dialkyl carbamate prodrugs were more stable in buffer and in plasma than N-monoalkyl carbamate prodrugs. In conclusion, N-monoalkyl carbamate prodrugs of NTX improved the systemic delivery of NTX across human skin in vitro. N,N-Dialkyl substitution in the prodrug moiety decreased skin permeation and plasma hydrolysis to the parent drug. The cross-sectional area of the carbamate head group was the major determinant of flux of the N-monoalkyl and N,N-dialkyl carbamate prodrugs of NTX.

Safety assessment of personal care products/cosmetics and their ingredients

We attempt to review the safety assessment of personal care products (PCP) and ingredients that are representative and pose complex safety issues. PCP are generally applied to human skin and mainly produce local exposure, although skin penetration or use in the oral cavity, on the face, lips, eyes and mucosa may also produce human systemic exposure. In the EU, US and Japan, the safety of PCP is regulated under cosmetic and/or drug regulations. Oxidative hair dyes contain arylamines, the most chemically reactive ingredients of PCP. Although arylamines have an allergic potential, taking into account the high number of consumers exposed, the incidence and prevalence of hair dye allergy appears to be low and stable. A recent (2001) epidemiology study suggested an association of oxidative hair dye use and increased bladder cancer risk in consumers, although this was not confirmed by subsequent or previous epidemiologic investigations. The results of genetic toxicity, carcinogenicity and reproductive toxicity studies suggest that modern hair dyes and their ingredients pose no genotoxic, carcinogenic or reproductive risk. Recent reports suggest that arylamines contained in oxidative hair dyes are N-acetylated in human or mammalian skin resulting in systemic exposure to traces of detoxified, i.e. non-genotoxic, metabolites, whereas human hepatocytes were unable to transform hair dye arylamines to potentially carcinogenic metabolites. An expert panel of the International Agency on Research of Cancer (IARC) concluded that there is no evidence for a causal association of hair dye exposure with an elevated cancer risk in consumers. Ultraviolet filters have important benefits by protecting the consumer against adverse effects of UV radiation; these substances undergo a stringent safety evaluation under current international regulations prior to their marketing. Concerns were also raised about the safety of solid nanoparticles in PCP, mainly TiO(2) and ZnO in sunscreens. However, current evidence suggests that these particles are non-toxic, do not penetrate into or through normal or compromised human skin and, therefore, pose no risk to human health. The increasing use of natural plant ingredients in personal care products raised new safety issues that require novel approaches to their safety evaluation similar to those of plant-derived food ingredients. For example, the Threshold of Toxicological Concern (TTC) is a promising tool to assess the safety of substances present at trace levels as well as minor ingredients of plant-derived substances. The potential human systemic exposure to PCP ingredients is increasingly estimated on the basis of in vitro skin penetration data. However, new evidence suggests that the in vitro test may overestimate human systemic exposure to PCP ingredients due to the absence of metabolism in cadaver skin or misclassification of skin residues that, in vivo, remain in the stratum corneum or hair follicle openings, i.e. outside the living skin. Overall, today's safety assessment of PCP and their ingredients is not only based on science, but also on their respective regulatory status as well as other issues, such as the ethics of animal testing. Nevertheless, the record shows that today's PCP are safe and offer multiple benefits to quality of life and health of the consumer. In the interest of all stakeholders, consumers, regulatory bodies and producers, there is an urgent need for an international harmonization on the status and safety requirements of these products and their ingredients.

Cosmeceutical vehicles

Consumers will pay a premium for high-performance skin and hair care products. The demand exists, and in return for the high cost, consumers expect the product to perform as claimed and to meet aesthetic standards beyond many products found in the mass market. To be successful in this highly competitive market, products must function as claimed or consumers will not repurchase. Effective contemporary high-end products must be properly formulated in nonirritating vehicles that consumers will perceive as elegant.

Dynamics and control of percutaneous drug absorption in the presence of epidermal turnover

The integration of epidermal turnover into the study of transdermal drug-delivery kinetics is addressed in light of classical control theory. A mathematical representation of the process, which includes Fickian diffusion and advection, was formulated in the frequency domain. This transformation facilitated a detailed analysis of the system dynamics and revealed the intricate relationships among a medicament transient absorption through the skin, the epidermal turnover rate, its physicochemical properties and the amount of drugs in a reservoir. The process, represented by transcendental transfer functions, was reduced to a second-order system with dead-time by minimizing the squared magnitude of the complex error between the original and simplified models. Clinically relevant parameters, such as the time to reach steady-state flux or drug concentration in the skin layers, are readily available from the low-order models. The time it takes to deliver a specified dose of drug to a particular depth in the skin is a function of the penetration depth and the diffusion coefficients of the drug molecules in the stratum corneum and the viable epidermis. An optimum administration protocol was developed for the transdermal delivery of chemicals when epidermal turnover is likely to affect their absorption into the systemic circulation.

A comparison of the fit of flux through hairless mouse skin from water data to three model equations

Data for the delivery of total species containing parent drugs from water through hairless mouse skin by prodrugs, logJ(MMAQ), has been fitted to the Roberts-Sloan, RS, the Kasting-Smith-Cooper, KSC, and Magnusson-Anissimov-Cross-Roberts, MACR, equations. The RS model which contains a parameter for the dependence of flux on solubility in water, S(AQ), as well as solubility in the lipid isopropyl myristate, S(IPM), gave the best fit: logJ(MMAQ)=-2.30+0.575 logS(IPM)+0.425 logS(AQ)-0.0016MW, r(2)=0.903. The values for the coefficients to the parameters are quite similar to those obtained when the RS model was fit to flux of solutes from water through human skin, logJ(MHAQ). There was no trend in predicting the under or over-performance of prodrugs based on their fit to the RS model and whether they were more or less soluble than their parent drugs. There was an inverse dependence of logJ(MMAQ) on partition coefficients or permeability coefficients similar to that observed for logJ(MHAQ). The similarities in trends for results for logJ(MMAQ) and logJ(MHAQ) suggests that design directives obtained from mouse skin can be extended to design new prodrugs or select new drugs for delivery through human skin.

Lack of effect of butylparaben and methylparaben on the reproductive system in male rats

BACKGROUND

Parabens are widely used preservatives in cosmetics and pharmaceutical products, and approved as food additives. Parabens have been considered safe for these uses for many years. Recently, adverse effects on male reproductive parameters in rats have been reported when parabens were given orally for 8 weeks starting at three weeks of age. Our studies used two representative parabens, methyl- and butylparaben, to try to replicate these studies and thereby evaluate potential reproductive effects in male Wistar rats.

METHODS

Diets containing 0, 100, 1000 or 10,000 ppm of either butyl- or methylparaben were fed to male rats for eight weeks. Rats were 22 days of age at the start of exposure. Parameters evaluated included organ weights, histopathology of reproductive tissues, sperm production, motility, morphology and reproductive hormone levels (butylparaben only).

RESULTS

None of the parameters evaluated for either paraben showed compound- or dosage-dependent adverse effects. Metabolism experiments of butylparaben indicate that it is rapidly metabolized by non-specific esterases to p-hydroxybenzoic acid and butanol, neither of which is estrogenic.

CONCLUSIONS

Exposure to methyl- or butylparaben in the diet for eight weeks did not affect any male reproductive organs or parameters at exposures as high as 10,000 ppm, corresponding to a mean daily dose of 1,141.1+/-58.9 or 1,087.6+/-67.8 mg/kg/day for methyl- and butylparaben, respectively. The rapid metabolism of parabens by esterases probably explains why these weakly estrogenic substances elicit no in vivo effects when administered by relevant exposure routes (i.e., topical and oral).

Final Amended Report on the Safety Assessment of Methylparaben, Ethylparaben, Propylparaben, Isopropylparaben, Butylparaben, Isobutylparaben, and Benzylparaben as used in Cosmetic Products

Parabens is the name given to a group of p-hydroxybenzoic acid (PHBA) esters used in over 22,000 cosmetics as preservatives at concentrations up to 0.8% (mixtures of parabens) or up to 0.4% (single paraben). The group includes Methylparaben, Ethylparaben, Propylparaben, Isopropylparaben, Butylparaben, Isobutylparaben, and Benzylparaben. Industry estimates of the daily use of cosmetic products that may contain parabens were 17.76 g for adults and 378 mg for infants. Parabens in cosmetic formulations applied to skin penetrate the stratum corneum in inverse relation to the ester chain length. Carboxylesterases hydrolyze parabens in the skin. Parabens do not accumulate in the body. Serum concentrations of parabens, even after intravenous administration, quickly decline and remain low. Acute toxicity studies in animals indicate that parabens are not significantly toxic by various routes of administration. Subchronic and chronic oral studies indicate that parabens are practically nontoxic. Numerous genotoxicity studies, including Ames testing, dominant lethal assay, hostmediated assay, and cytogenic assays, indicate that the Parabens are generally nonmutagenic, although Ethylparaben and Methylparaben did increase chromosomal aberrations in a Chinese Hamster ovary cell assay. Ethylparaben, Propylparaben, and Butylparaben in the diet produced cell proliferation in the forestomach of rats, with the activity directly related to chain length of the alkyl chain, but Isobutylparaben and Butylparaben were noncarcinogenic in a mouse chronic feeding study. Methylparaben was noncarcinogenic when injected subcutaneously in mice or rats, or when administered intravaginally in rats, and was not cocarcinogenic when injected subcutaneously in mice. Propylparaben was noncarcinogenic in a study of transplacental carcinogenesis. Methylparaben was nonteratogenic in rabbits, rats, mice, and hamsters, and Ethylparaben was nonteratogenic in rats. Parabens, even at levels that produce maternal toxicity, do not produce fetal anomalies in animal studies. Parabens have been extensively studied to evaluate male reproductive toxicity. In one in vitro study, sperm were not viabile at concentrations as low as 6 mg/ml Methylparaben, 8 mg/ml Ethylparaben, 3 mg/ml Propylparaben, or 1 mg/ml Butylparaben, but an in vivo study of 0.1% or 1.0% Methylparaben or Ethylparaben in the diet of mice reported no spermatotoxic effects. Propylparaben did affect sperm counts at all levels from 0.01% to 1.0%. Epididymis and seminal vesicle weight decreases were reported in rats given a 1% oral Butylparaben dose; and decreased sperm number and motile activity in F1 offspring of rats maternally exposed to 100 mg/kg day–1 were reported. Decreased sperm numbers and activity were reported in F1 offspring of female rats given Butylparaben (in DMSO) by subcutaneous injection at 100 or 200 mg/kg day–1, but there were no abnormalities in the reproductive organs. Methylparaben was studied using rats at levels in the diet up to an estimated mean dose of 1141.1 mg/kg day–1 with no adverse testicular effects. Butylparaben was studied using rats at levels in the diet up to an estimated mean dose of 1087.6 mg/kg day–1 in a repeat of the study noted above, but using a larger number of animals and a staging analysis of testicular effects—no adverse reproductive effects were found. Butylparaben does bind to estrogen receptors in isolated rat uteri, but with an affinity orders of magnitude less than natural estradiol. Relative binding (diethylstilbesterol binding affinity set at 100) to the human estrogen receptors α and β increases as a function of chain length from not detectable for Methylparaben to 0.267 ± 0.027 for human estrogen receptor α and 0.340 ± 0.031 for human estrogen receptor β for Isobutylparaben. In a study of androgen receptor binding, Propylparaben exhibited weak competitive binding, but Methylparaben had no binding effect at all. PHBA at 5 mg/kg day–1 subcutaneously (s.c.) was reported to produce an estrogenic response in one uterotrophic assay using mice, but there was no response in another study using rats (s.c. up to 5 mg/kg day–1) and mice (s.c. up to 100 mg/kg day–1) and in a study using rats (s.c. up to 100 mg/kg day–1). Methylparaben failed to produce any effect in uterotrophic assays in two laboratories, but did produce an effect in other studies from another laboratory. The potency of Methylparaben was at least 1000 × less when compared to natural estradiol. The same pattern was reported for Ethylparaben, Propylparaben, and Butylparaben when potency was compared to natural estradiol. In two studies, Isobutylparaben did produce an estrogenic response in the uterotrophic assay, but the potency was at least 240,000 × less than estradiol. In one study, Benzylparaben produced an estrogenic response in the uterotrophic assay, but the potency was at least 330,000 × less than estradiol. Estrogenic activity of parabens and PHBA was increased in human breast cancer cells in vitro, but the increases were around 4 orders of magnitude less than that produced by estradiol. Parabens are practically nonirritating and nonsensitizing in the population with normal skin. Paraben sensitization has occurred and continues to be reported in the case literature, but principally when exposure involves damaged or broken skin. Even when patients with chronic dermatitis are patch-tested to a parabens mix, parabens generally induce sensitization in less than 4% of such individuals. Many patients sensitized to paraben-containing medications can wear cosmetics containing these ingredients with no adverse effects. Clinical patch testing data available over the past 20 years demonstrate no significant change in the overall portion of dermatitis patients that test positive for parabens. As reviewed by the Cosmetic Ingredient Review (CIR) Expert Panel, the available acute, subchronic, and chronic toxicity tests, using a range of exposure routes, demonstrate a low order of parabens' toxicity at concentrations that would be used in cosmetics. Parabens are rarely irritating or sensitizing to normal human skin at concentrations used in cosmetics. Although parabens do penetrate the stratum corneum, metabolism of parabens takes place within viable skin, which is likely to result in only 1% unmetabolized parabens available for absorption into the body. The Expert Panel did consider data in the category of endocrine disruption, including male reproductive toxicity and various estrogenic activity studies. The CIR Expert Panel compared exposures to parabens resulting from use of cosmetic products to a no observed adverse effect level (NOAEL) of 1000 mg/kg day–1 based on the most statistically powerful and well-conducted study of the effects of Butylparabens on the male reproductive system. The CIR Expert Panel considered exposures to cosmetic products containing a single parabens preservative (use level of 0.4%) separately from products containing multiple parabens (use level of 0.8%) and infant exposures separately from adult exposures in determining margins of safety (MOS). The MOS for infants ranged from ~6000 for single paraben products to ~3000 for multiple paraben products. The MOS for adults ranged from 1690 for single paraben products to 840 for multiple paraben products. The Expert Panel considers that these MOS determinations are conservative and likely represent an overestimate of the possibility of an adverse effect (e.g., use concentrations may be lower, penetration may be less) and support the safety of cosmetic products in which parabens preservatives are used.

Evaluation of simultaneous permeation and metabolism of methyl nicotinate in human, snake, and shed snake skin

The transdermal permeation and metabolic characteristics of methyl nicotinate (MN) in stratum corneum and split-thickness human skin and three species of shed snake and snake skin (Elaphae obsoleta, Naja kaouthia, and Python molurus bivittatus) were evaluated. In vitro skin transport using excised skin and hydrolysis experiments using skin homogenate were carried out. The flux of MN, a metabolite, nicotinic acid (NA), and the total (MN+NA), as well as kinetic parameters (V(max) and K(m)) for hydrolysis of MN were determined and compared among various skin types. The total flux from MN-saturated solution through human skin was not significantly different from that through snake and shed snake skin of Elaphae obsoleta, Naja kaouthia but was significantly higher than that through snake and shed snake skin of Naja kaouthia (p < 0.05). A great difference in skin esterase activity was observed between human and snake in both snake skin and shed snake skin of all species. In all skins except the stratum corneum of human skin, NA flux increased with an increase in MN donor concentration and reached a plateau, suggesting that metabolic saturation was taking place in the skin. NA flux at the plateau and MN donor concentrations at which the NA flux reached a plateau also varied by species. These findings indicated that the discrepancy in transdermal profiles of MN among skins tested was predominantly due to the difference in the esterase activity in the skin.

Derivatives of 5-Aminolevulinic Acid for Photodynamic Therapy

Photodynamic therapy (PDT) is a clinical treatment that combines the effects of visible light irradiation with subsequent biochemical events that arise from the presence of a photosensitising drug (possessing no dark toxicity) to cause destruction of selected cells. Today, the most common agent used in dermatological PDT is 5-aminolevulinic acid (ALA). As a result of its hydrophilic character, ALA penetrates skin lesions poorly when applied topically. Its systemic bioavailability is limited and it is known to cause significant side effects when given orally or intravenously. Numerous chemical derivatives of ALA have been synthesised with the aims of either improving topical penetration or enhancing systemic bioavailability, while reducing side effects. In vitro cell culture experiments with ALA derivatives have yielded promising results. However, if ALA derivatives are to demonstrate meaningful clinical benefits, a rational approach to topical formulation design is required, along with a systematic study aimed at uncovering the true potential of ALA derivatives in photodynamic therapy. With respect to systemic ALA delivery, more study is required in the developing area of ALA-containing dendrons and dendrimers.

Assessment of principal parabens used in cosmetics after their passage through human epidermis-dermis layers (ex-vivo study)

Concern is continuously raised about the safety of parabens which are present in most of the cosmetic preparations. In this investigation, methyl-, ethyl-, propyl- and butyl paraben (MP, EP, PP, BP), in a commercial cosmetic lotion, were deposited on human skin fragments, collected after surgical operations. Permeated parabens were determined after their passage through human epidermis-dermis layers, fixed on Franz diffusion cells. Bovine serum albumin (3%) was employed as receptor fluid. Then, parabens were assessed by liquid chromatography. The objective of this research was to determine the permeation of these molecules through human epidermis-dermis layers, and their possible passage to body tissues and/or accumulation in skin layers. Two groups of experiments were performed. In the first experimental group (G1), unique doses of the cosmetic were deposited on skin fragments fixed on Franz cells (n = 6), at time 0 h, followed with different withdrawn times of the receptor fluid at 12, 24 and 36 h. G1 results demonstrated that parabens penetration was influenced by their lipophilicity: more lipophilic the parabens were (BP > PP > EP > MP), less they crossed the skin layers (BP < PP < EP < MP). The second experimental group (G2) was constituted of three equal deposits on each Franz cell (n = 6) at different hour times 0, 12 and 24 h followed with three withdrawn times of the receptor fluid at 12, 24 and 36 h. The G2 results indicated that investigated parabens had significant increasing permeations in skin layers. This situation provokes the accumulation of these molecules which were considered by some authors as the cause of skin toxicities and carcinogenicity.

Parabens as urinary biomarkers of exposure in humans

BACKGROUND

Parabens appear frequently as antimicrobial preservatives in cosmetic products, in pharmaceuticals, and in food and beverage processing. In vivo and in vitro studies have revealed weak estrogenic activity of some parabens. Widespread use has raised concerns about the potential human health risks associated with paraben exposure.

OBJECTIVES

Assessing human exposure to parabens usually involves measuring in urine the conjugated or free species of parabens or their metabolites. In animals, parabens are mostly hydrolyzed to p-hydroxybenzoic acid and excreted in the urine as conjugates. Still, monitoring urinary concentrations of p-hydroxybenzoic acid is not necessarily the best way to assess exposure to parabens. p-hydroxybenzoic acid is a nonspecific biomarker, and the varying estrogenic bioactivities of parabens require specific biomarkers. Therefore, we evaluated the use of free and conjugated parent parabens as new biomarkers for human exposure to these compounds.

RESULTS

We measured the urinary concentrations of methyl, ethyl, n-propyl, butyl (n- and iso-), and benzyl parabens in a demographically diverse group of 100 anonymous adults. We detected methyl and n-propyl parabens at the highest median concentrations (43.9 ng/mL and 9.05 ng/mL, respectively) in nearly all (> 96%) of the samples. We also detected other parabens in more than half of the samples (ethyl, 58%; butyl, 69%). Most important, however, we found that parabens in urine appear predominantly in their conjugated forms.

CONCLUSIONS

The results, demonstrating the presence of urinary conjugates of parabens in humans, suggest that such conjugated parabens could be used as exposure biomarkers. Additionally, the fact that conjugates appear to be the main urinary products of parabens may be important for risk assessment.

Topical Delivery of Retinyl Ascorbate Co-Drug

Chemical and enzymatic hydrolysis of the co-drug of retinoic acid (vitamin A) and ascorbic acid (vitamin C) - retinyl ascorbate (RA-AsA)--have been studied. Firstly, the amount of protein and ester hydrolysis activity was determined in crude cellular extracts from freshly excised porcine ear skin (<3 h) and stored porcine ear skin (frozen >6 months) using ethyl butyrate as model substrate. The stability of RA-AsA was then determined in the crude cell extracts with and without additional antioxidants. Lastly, the enzymatic hydrolysis of RA-AsA and retinyl-2-carboxy-2-hydroxy-ethanoate were determined by incubating with porcine liver esterase - retinol palmitate and ascorbyl palmitate were included for comparison. Freshly excised skin contained higher amounts of active proteins than previously frozen skin. RA-AsA underwent hydrolytic reduction causing the AsA moiety to disintegrate due to the presence of free radicals in the media. An intermediate was produced that seemed to be cleaved by enzymes. Addition of ascorbic acid, as antioxidant, to the media of crude protein extracts decelerated the hydrolysis rate. This was supported when RA-AsA and retinyl-2-carboxy-2-hydroxy-ethanoate were incubated separately with pure esterase. There was approximately 5-fold more soluble protein per ml of cytosol in the fresh skin compared to the stored skin. Therefore, the amount of protein present within approximately 1.5 cm(2) of skin (average diffusion area in the Franz cells used in our skin penetration studies) was 0.06 mg cm(-2) and 0.01 mg cm(-2) for fresh and stored extracts, respectively.

The impact of skin viability on drug metabolism and permeation—BSA toxicity on primary keratinocytes

For testing cutaneous absorption of drugs, ingredients of cosmetics and also for risk assessment of industrial compounds predictable in vitro test protocols are under investigation using excised skin or reconstructed human epidermis. Since the metabolizing enzymes expressed by viable skin can influence the absorption behaviour of substances by changing their structure and thereby their physicochemical characteristics, the metabolic capacity should be considered in the design of the test protocols of compounds susceptible to metabolism. Then data, generated using viable reconstructed epidermis may reflect the in vivo situation. Interestingly, bovine serum albumin (BSA) commonly used in receptor media in permeation studies to facilitate solubility of highly lipophilic substances strongly inhibited the metabolism of topically applied prednicarbate in reconstructed epidermis. Here, we show that 5% BSA is toxic to reconstructed epidermis and keratinocytes which was consistent with the earlier findings. While media toxicity (deficiency media) was at least partly the cause of both apoptotic and necrotic processes in keratinocytes, BSA only slightly increased the rate of necrotic cells. Moreover, caspase inhibitors did not reduce BSA toxicity. Yet, the results show that BSA toxicity on keratinocytes has to be carefully considered if this protein is used in permeation studies with reconstructed epidermis.

Regulation of endocrine-disrupting chemicals: critical overview and deficiencies in toxicology and risk assessment for human health

Regulation of endocrine-disrupting chemicals is reviewed in terms of hazard assessment (regulatory toxicology) and risk assessment. The current range of regulatory general toxicology protocols can detect endocrine toxicity, but specific endocrine toxicology tests are required to confirm mechanisms (e.g. oestrogenic, anti-androgenic). Strategies for validating new endocrine toxicology protocols and approaches to data assessment are discussed, and deficiencies in regulatory toxicology testing (e.g. lack of adrenocortical function assessment) identified. Recent evidence of a role of prolactin in human breast cancer also highlights deficiencies in regulatory evaluation. Actual human exposure to chemicals and the high-exposure example of chemicals in body-care cosmetics is reviewed with reference to evidence that common ingredients (e.g. parabens, cyclosiloxanes) are oestrogenic. The hypothesis and epidemiology concerning chemical exposure from body-care cosmetics (moisturizers, lotions, sun screens, deodorants) and breast cancer in women is reviewed, applying Bradford-Hill criteria for association and causality, and research requirements are identified.

Environmental oestrogens, cosmetics and breast cancer

The established role of oestrogen in the development and progression of breast cancer raises questions concerning a potential contribution from the many chemicals in the environment which can enter the human breast and which have oestrogenic activity. A range of organochlorine pesticides and polychlorinated biphenyls possess oestrogen-mimicking properties and have been measured in human breast adipose tissue and in human milk. These enter the breast from varied environmental contamination of food, water and air, and due to their lipophilic properties can accumulate in breast fat. However, it is emerging that the breast is also exposed to a range of oestrogenic chemicals applied as cosmetics to the underarm and breast area. These cosmetics are left on the skin in the appropriate area, allowing a more direct dermal absorption route for breast exposure to oestrogenic chemicals and allowing absorbed chemicals to escape systemic metabolism. This review considers evidence in support of a functional role for the combined interactions of cosmetic chemicals with environmental oestrogens, pharmacological oestrogens, phyto-oestrogens and physiological oestrogens in the rising incidence of breast cancer.

5-Aminolevulinic Acid Derivatives in Photomedicine: Characteristics, Application and Perspectives

The introduction of lipophilic derivatives of the naturally occurring heme precursor 5-aminolevulinic acid (5-ALA) into photomedicine has led to a true revival of this research area. 5-ALA-mediated photodynamic therapy (PDT) and fluorescence photodetection (FD) of neoplastic disease is probably one of the most selective cancer treatments currently known in oncology. To date, this method has been assessed experimentally for the treatment of various medical indications. However, the limited local bioavailability of 5-ALA has widely prevented its use in daily clinical practice. Although researchers were already aware of this drawback early during the development of 5-ALA-mediated PDT, only recently have well-established concepts in pharmaceutical science been adapted to investigate ways to overcome this drawback. Recently, two derivatives of 5-ALA, methylaminolevulinate (MAL) and hexylaminolevulinate (HAL), gained marketing authorization from the regulatory offices in Europe and Australia. MAL is marketed under the trade name Metvix for the treatment of actinic keratosis and difficult-to-treat basal cell carcinoma. HAL has recently been launched under the trade name Hexvix to improve the detection of superficial bladder cancer in Europe. This review will first present the fundamental concepts underlying the use of 5-ALA derivatives in PDT and FD from a chemical, biochemical and pharmaceutical point of view. Experimental evidences from preclinical data on the improvements and limits observed with 5-ALA derivatives will then be introduced. The state-of-the-art from clinical studies with 5-ALA esters will be discussed, with special emphasis placed on the process that led to the development of MAL in dermatology and to HAL in urology. Finally, we will discuss promising medical fields in which use of 5-ALA derivatives might potentially lead to further use of this methodology in photomedicine.

Bioluminescent yeast assays for detecting estrogenic and androgenic activity in different matrices

In this paper we describe the construction and use of a set of bioluminescent yeast strains for the detection of compounds that can affect androgen or estrogen receptor mediated hormonal signalling. The set includes Saccharomyces cerevisiae strains expressing human androgen receptor (AR), estrogen receptor alpha (ERalpha) or estrogen receptor beta (ERbeta), along with firefly luciferase controlled by a respective hormone responsive promoter. A constitutively luminescent strain was included in the set for determining the cytotoxicity of the sample. Yeast cells were incubated with pure chemicals or complex samples for 2.5 h, after which the signal could be detected from the cell-sample mixture after simply adding the D-luciferin substrate. The assays could be completed in one day and they required no cell lysis or centrifugation steps, which makes them suitable for high-throughput analysis of samples. Due to a short incubation time the assays are directly applicable to different sample matrices, requiring no pretreatment of the samples. The assays were used to assess the hormonal activity in moisturizing lotions as an example of a complex sample matrix known to contain endocrine disrupting chemicals. Six out of eight tested moisturisers showed high estrogenic activity, whereas no androgenic activity was observed in the samples.

A review of the endocrine activity of parabens and implications for potential risks to human health

Parabens are a group of the alkyl esters of p-hydroxybenzoic acid and typically include methylparaben, ethylparaben, propylparaben, butylparaben, isobutylparaben, isopropylparaben, and benzylparaben. Parabens (or their salts) are widely used as preservatives in cosmetics, toiletries, and pharmaceuticals due to their relatively low toxicity profile and a long history of safe use. Testing of parabens has revealed to varying degrees that individual paraben compounds have weakly estrogenic activity in some in vitro screening tests, such as ligand binding to the estrogen receptor, regulation of CAT gene expression, and proliferation of MCF-7 cells. Reported in vivo effects include increased uterine weight (i.e., butyl-, isobutyl-, and benzylparaben) and male reproductive-tract effects (i.e., butyl- and propylparaben). However, in relation to estrogen as a control during in vivo studies, the parabens with activity are many orders of magnitude less active than estrogen. While exposure to sufficient doses of exogenous estrogen can increase the risk of certain adverse effects, the presumption that similar risks might also result from exposure to endocrine-active chemicals (EACs) with far weaker activity is still speculative. In assessing the likelihood that exposure to weakly active EACs might be etiologically associated with adverse effects due to an endocrine-mediated mode of action, it is paramount to consider both the doses and the potency of such compounds in comparison with estrogen. In this review, a comparative approach involving both dose and potency is used to assess whether in utero or adult exposure to parabens might be associated with adverse effects mediated via an estrogen-modulating mode of action. In utilizing this approach, the paraben doses required to produce estrogenic effects in vivo are compared with the doses of either 17beta-estradiol or diethylstilbestrol (DES) that are well established in their ability to affect endocrine activity. Where possible and appropriate, emphasis is placed on direct comparisons with human data with either 17beta-estradiol or DES, since this does not require extrapolation from animal data with the uncertainties inherent in such comparisons. Based on these comparisons using worst-case assumptions pertaining to total daily exposures to parabens and dose/potency comparisons with both human and animal no-observed-effect levels (NOELs) and lowest-observed-effect levels (LOELs) for estrogen or DES, it is biologically implausible that parabens could increase the risk of any estrogen-mediated endpoint, including effects on the male reproductive tract or breast cancer. Additional analysis based on the concept of a hygiene-based margin of safety (HBMOS), a comparative approach for assessing the estrogen activities of weakly active EACs, demonstrates that worst-case daily exposure to parabens would present substantially less risk relative to exposure to naturally occurring EACs in the diet such as the phytoestrogen daidzein.

Safety assessment of esters of p-hydroxybenzoic acid (parabens)

Parabens are widely used as preservatives in food, cosmetic and pharmaceutical products. Acute, subchronic, and chronic studies in rodents indicate that parabens are practically non-toxic. Parabens are rapidly absorbed, metabolized, and excreted. In individuals with normal skin, parabens are, for the most part, non-irritating and non-sensitizing. However, application of compounds containing parabens to damaged or broken skin has resulted in sensitization. Genotoxicity testing of parabens in a variety of in vitro and in vivo studies primarily gave negative results. The paraben structure is not indicative of carcinogenic potential, and experimental studies support these observations. Some animal studies have reported adverse reproductive effects of parabens. In an uterotrophic assay, methyl and butyl paraben administered orally to immature rats were inactive, while subcutaneous administration of butyl paraben produced a weak positive response. The ability of parabens to transactivate the estrogen receptor in vitro increases with alkyl group size. The detection of parabens in a small number of breast tumor tissue samples and adverse reproductive effects of parabens in animals has provoked controversy over the continued use of these substances. However, the possible estrogenic hazard of parabens on the basis of the available studies is equivocal, and fails to consider the metabolism and elimination rates of parabens, which are dose, route, and species dependent. In light of the recent controversy over the estrogenic potential of parabens, conduct of a reproductive toxicity study may be warranted.

In Vitro/in Vivo Correlation of Transdermal Naltrexone Prodrugs in Hairless Guinea Pigs

PURPOSE

The purpose of this investigation was to evaluate the in vitro and in vivo percutaneous absorption of the following prodrugs of naltrexone (NTX): 2'-ethylbutyryl-3-O-ester-NTX (ETBUT-ester), methyl-3-O-carbonate-NTX (ME-carbonate), ethyl-3-O-carbamate-NTX (ET-carbamate), and N,N-dimethyl-3-O-carbamate-NTX (DME-carbamate) in hairless guinea pigs.

METHODS

In vitro fluxes of NTX and its prodrugs through guinea pig skin were determined using a flow-through diffusion cell system. The pharmacokinetics of NTX prodrugs were determined after topical application of transdermal patches in guinea pigs.

RESULTS

All the prodrugs hydrolyzed to NTX on passing through the skin, and ME-carbonate provided the highest NTX flux and had the highest apparent permeability coefficient (K(p)). ME-carbonate and ET-carbamate underwent the highest extent of bioconversion to NTX upon passing through the skin as compared to ETBUT-ester and DME-carbamate. The results of the in vivo studies indicated that a significant amount of NTX was delivered after the application of transdermal patches of NTX prodrugs. A mean steady-state plasma concentration of 7.1 ng/ml was obtained after the application of transdermal patches of ME-carbonate. A good correlation was obtained between the in vitro and in vivo results.

CONCLUSIONS

The results of the in vivo studies indicated that the ME-carbonate prodrug of NTX was the most promising drug candidate for transdermal delivery.

Oestrogenic activity ofp-hydroxybenzoic acid (common metabolite of paraben esters) and methylparaben in human breast cancer cell lines

This paper addresses the question of whether p-hydroxybenzoic acid, the common metabolite of parabens, possesses oestrogenic activity in human breast cancer cell lines. The alkyl esters of p-hydroxybenzoic acid (parabens) are used widely as preservatives in consumer products to which the human population is exposed and have been shown previously to possess oestrogenic activity and to be present in human breast tumour tissue, which is an oestrogen-responsive tissue. Recent work has shown p-hydroxybenzoic acid to give an oestrogenic response in the rodent uterotrophic assay. We report here that p-hydroxybenzoic acid possesses oestrogenic activity in a panel of assays in human breast cancer cell lines. p-Hydroxybenzoic acid was able to displace [(3)H]oestradiol from cytosolic oestrogen receptor of MCF7 human breast cancer cells by 54% at 5 x 10(6)-fold molar excess and by 99% at 10(7)-fold molar excess. It was able to increase the expression of a stably integrated oestrogen responsive reporter gene (ERE-CAT) at a concentration of 5 x 10(-4) M in MCF7 cells after 24 h and 7 days, which could be inhibited by the anti-oestrogen ICI 182 780 (Faslodex, fulvestrant). Proliferation of two human breast cancer cell lines (MCF7, ZR-75-1) could be increased by 10(-5) M p-hydroxybenzoic acid. Following on from previous studies showing a decrease in oestrogenic activity of parabens with shortening of the linear alkyl chain length, this study has compared the oestrogenic activity of p-hydroxybenzoic acid where the alkyl grouping is no longer present with methylparaben, which has the shortest alkyl group. Intrinsic oestrogenic activity of p-hydroxybenzoic acid was similar to that of methylparaben in terms of relative binding to the oestrogen receptor but its oestrogenic activity on gene expression and cell proliferation was lower than that of methylparaben. It can be concluded that removal of the ester group from parabens does not abrogate its oestrogenic activity and that p-hydroxybenzoic acid can give oestrogenic responses in human breast cancer cells.